Flexographic printing is a printing process which uses a flexible rubber or photopolymer relief plate, prepared with raised areas called relief areas, corresponding to an image to be printed, and surrounded by lower areas called floor areas. The plate is typically secured to a revolving drum which is contacted by an anilox roll system to coat the relief areas with ink and subsequently presses the inked areas against a moving sheet of, e.g., paper or corrugated box board. The flexible printing plates may alternatively be applied to a plate mount, e.g., hand stamps, or, potentially, non-planar surfaces other than drums.
Images for flexographic printing are typically created by using a negative of an image to mask image areas on a layer of photosensitive polymer (“photopolymer”). Photopolymers cure by cross-linking under exposure to ultraviolet (UV) light. The plates may be imaged digitally (what is known as computer-to-plate or CTP) or by the analog process of exposing and developing traditional film. Such films (as supplied by Kodak, Fuji, etc.) are processed into negatives via an image setter.
Another method of making negatives uses an inkjet printer to print a UV-blocking ink onto a clear plastic, e.g., polyester, material. The plastic material may be coated with an inkjet receptive coating, e.g., a micro-porous coating, or treated in some other manner to produce an inkjet-receptive surface such as acid etching, etc., to allow the ink to adhere to the film and dry quickly without migrating. The plastic material may alternatively be printed with an inkjet ink that is both compatible with the plastic surface itself and forms an acceptably stable and detailed image.
Liquid photopolymer flexographic plates are made in an exposure unit having a horizontal bottom glass with a source of UV light below it (lower light), and a lid having a flat top glass with a source of UV light above it (upper light). A traditional flexographic plate is made in the following manner: a negative of the desired image, as prepared by one of the above methods, is placed on the bottom glass, the negative is protected by a thin optically clear film known as cover film. Liquid photopolymer is then cast over the cover film to a predetermined thickness. A polyester backing sheet known as a substrate is then laminated over the liquid photopolymer. Thus a sandwich of negative, cover film, liquid polymer and substrate is created. The substrate has an adhesive coating called “tie-coat.” This allows the cured photopolymer to adhere or bond to the polyester. The substrate is placed in such a manner that the tie-coat is in direct contact with the liquid photopolymer. The sheet photopolymers (such as supplied by DuPont or the Flint Group) comprise a layer of photopolymer pre-applied to a polyester substrate, whereas the liquid photopolymers (such as supplied by Chemence and MacDermid) are cast over the image negative (protected by a cover film) and bond to a polyester substrate by the tie-coat. (Sheet photopolymers have a tie-coat pre-applied to the polyester before the photopolymer layer is added.) Sheet photopolymer material may be imaged on dual light source equipment as described above or by a single light source. In either case, the floor is first created by exposing the photopolymer through the substrate. The photographic negative is then placed on the opposite surface of the photopolymer, held in intimate contact through the application of a vacuum sheet, and imaged through it.
Precise reproduction of the negative image detail onto the photopolymer requires that the negative be placed as close to the photopolymer layer as possible. In the liquid plate-making process the bottom glass surface is grooved and etched in such a way that, when a vacuum is applied to the grooves, air is removed between the cover film and the relief image negative. In the sheet process a vacuum sheet is applied over the negative and photopolymer to remove air between the negative and the photopolymer.
Next, the UV light source in the lid (the upper light) is turned on for a prescribed amount of time to cause the photopolymer adjacent to the substrate to cross-link uniformly over the entire plate, forming the floor. The areas to be imaged are then exposed by the lower UV light (from below the bottom glass) that shines through the clear areas of the relief image negative, which causes the photosensitive polymer to cross-link, forming images that bond to the polymer floor. The (liquid) polymer that is not exposed to UV light remains in a liquid state to be later reclaimed and reused. Unexposed sheet photopolymer, by contrast, is not reusable and becomes waste.
The plate-making arrangement is then taken out of the exposure unit and the photopolymer material that has not been cured by UV light is removed from it, leaving the areas exposed to light as relief areas. When sheet photopolymers are used, a combination of solvent and mechanical action is typically used to remove the un-polymerized material from the sheet. The removed material is discarded. Un-polymerized liquid photopolymers, on the other hand, have the advantage of being reclaimable for reuse, because the un-polymerized material is not diluted by solvent or contaminated with bits of polymerized material. They can be reclaimed by draining and wiping the un-polymerized liquid off the substrate into a suitable container. Photopolymer reclaim can be accomplished manually by squeegee, by a reclaim board with rollers or by an air knife and augmented or enhanced by using heat.
Typically the floor area on such traditional flexographic plates accounts for approximately one half to two thirds of the plate thickness, and the remainder is the relief area. The floor gives dimensional stability to the plate and provides support to the relief (imaged) area. Because so much of the photopolymer is used up in making a floor that covers the entire sheet, a method of making a flexographic plate has been devised to minimize the floor area and increase significantly the reclaim of liquid photopolymer. Printing plates made from this process are called Imposition Plates, Island Plates or I-Plates. Liquid photopolymer reclaim of 20% to 35% on a traditional printing plate can be increased to 55% to 65% on an I-plate.
I-plates are made by adding another step to the above process. Instead of making a floor that extends over the entire plate, a second photographic negative is placed on top of the photopolymer layer. This negative, called a masking film, is a negative that outlines the image areas on the negative with a border of one-quarter to one half inch, more or less. It is predominately black. The relief image negative and masking film are aligned so that each image area of the relief image negative is approximately in the middle of each clear area of the masking film. This prevents a floor area from being created where the masking film is black.
The most commonly-made flexographic printing plates and hand stamps are now manufactured as island plates in the following manner: the first exposure to upper UV light from the lid is through the masking negative, causing islands of cured polymer to be formed beginning in the photosensitive layer adjacent to the substrate. The timing and intensity of the exposure are limited to prevent the polymerization extending all the way through the photopolymer layer from the substrate to the free surface of the layer. The second lower UV exposure, from below the relief image negative, causes the cured detailed relief image to form on top of the islands.
Because of the placement of the second negative above the photopolymer substrate layer, the application of vacuum to remove air from between the negative and the photopolymer substrate layer is less effective than when just the substrate is used. Thus, in this prior art method for producing an I-plate, the increase in recovery of photopolymer comes at the expense of some loss of precision of the relief image. Alternatively the mask negative can be placed above the exposure glass but in this fashion distorts the resulting island formation. The “spread” in the light transmission through the thick upper glass yields an island that is larger than the negative with a broader shoulder. It is also more difficult to register the second negative to the first negative in setting up the process There is thus a need for a substrate and method for producing I-plates which improves recovery of photopolymer without loss of image quality.